If you spend any time in Lyme disease or Morgellons communities, you’ve probably heard that Bartonella is a common “co-infection” that comes from ticks. Many people assume that if they’ve had a tick bite and later test positive for Bartonella, the tick must have given it to them.
But what does the actual evidence say?
Researchers have now tested well over 10,000 ticks — specifically the kind of ticks that spread Lyme disease — using modern, highly sensitive testing methods. The results are surprising to many people: Bartonella was either completely absent or extremely rare.
This article breaks down what the research actually shows, why the idea that Bartonella comes from ticks became so common, and what it means for people trying to understand their symptoms.
What Is Bartonella?
Bartonella is a type of bacteria that can cause illness in humans. The most well-known form is called cat scratch disease, which usually causes swollen lymph nodes, fever, and fatigue after someone is scratched or bitten by an infected cat.
Other types of Bartonella can cause different symptoms, sometimes more serious ones, especially in people with weakened immune systems. The bacteria are real, and the infections they cause are real. The question is how people are actually getting them.
For decades, scientists have known that Bartonella is most commonly spread by fleas (especially cat fleas) and through cat scratches or bites. These are the proven, well-documented ways the bacteria move from animals to people.
Why Do So Many People Think Bartonella Comes from Ticks?
There are a few reasons this idea became popular:
- Many people with Lyme disease also test positive for Bartonella.
- Some patients develop symptoms that overlap with both conditions (fatigue, joint pain, neurological issues, etc.).
- Bartonella DNA has sometimes been found inside ticks.
- Certain doctors and patient communities began treating Bartonella as a common tick-borne co-infection.
Because of this overlap, it became common to assume that ticks were transmitting Bartonella the same way they transmit Lyme disease. Over time, this assumption spread widely in online communities and some treatment approaches.
But assumptions and actual evidence are two different things.
What Do Large Tick Studies Actually Show?
This is where the numbers become important.
Scientists have tested thousands of ticks across different studies using increasingly sensitive methods. Here’s what some of the largest and most recent research found:
- One major U.S. study tested 2,607 ticks (both nymphs and adults) collected from across many states. Using a sensitive test called a quadruplex PCR assay combined with next-generation sequencing, researchers found zero Bartonella in any of the ticks. View study
- Another U.S. study looked at 792 blood-fed ticks that had been removed from rodents. Even in these ticks that had recently fed, Bartonella was only found in one single larva. View study
- A large study in Germany tested approximately 6,700 ticks. Bartonella was also very uncommon. View study
When you add these studies together, researchers have now examined well over 10,000 ticks using modern testing methods. The consistent finding is that Bartonella is either not present or extremely rare in the ticks that most commonly bite humans in North America and Europe.
This doesn’t mean Bartonella never appears in any tick anywhere. It means that in large, well-designed studies of the ticks most relevant to human disease, Bartonella transmission by ticks has not been demonstrated.
A Better Way to Test for Bartonella
Part of the confusion comes from how Bartonella has been tested in the past. Older tests often looked for just one target and could miss some species or give unclear results.
In 2023, researchers at the CDC developed a more advanced test called a quadruplex PCR amplicon sequencing assay. This method looks for four different genetic targets at once and then uses next-generation sequencing to identify exactly which Bartonella species (if any) are present.
This newer approach is more accurate than older single-target tests. Even when this improved testing method was used on thousands of ticks, Bartonella was still rarely found.
Why Has the “Tick-Borne Bartonella” Idea Been So Persistent?
There are understandable reasons this belief continues:
- Symptom overlap: Bartonella and Lyme can cause similar symptoms, so it’s easy to assume they come from the same source.
- Testing limitations: Older tests weren’t always reliable, which sometimes led to confusion about what was actually being detected.
- Online communities: Once an idea spreads widely in patient groups, it can be difficult to shift, even when new research comes out.
- Clinical experience: Some practitioners see patients improve after treating for Bartonella and naturally assume the bacteria came from a tick.
While these factors are understandable, they don’t override the findings from large tick studies. Just because someone has both Lyme and Bartonella doesn’t automatically mean one tick gave them both.
What This Means If You’re Dealing with Symptoms
If you’re experiencing symptoms and have tested positive for Bartonella, it’s still important to work with a knowledgeable healthcare provider. Bartonella infections are real and may need treatment.
However, understanding how the bacteria are most likely transmitted can help you and your doctor focus on the most probable sources. In most cases, that points toward fleas or cat exposure rather than ticks.
This also matters for people exploring Morgellons or complex chronic symptoms. When information about co-infections is unclear or overstated, it can make it harder to get an accurate picture of what’s happening in the body.
The Bottom Line
Large studies involving over 10,000 ticks have found little to no evidence that the ticks responsible for Lyme disease are also significant carriers of Bartonella. The scientific evidence continues to show that Bartonella is primarily transmitted by fleas and cats — not ticks.
While research is always ongoing, current data does not support the widespread belief that Bartonella is a common tick-borne co-infection in the same way as other well-established tick-borne illnesses.
If you’re trying to understand your own health situation, focusing on the most evidence-based information gives you the best chance of getting clear answers.
